1. Field of the Invention
The present invention relates to technology for predicting crack propagation in the rotors of generators used in energy plant, determining the operation conditions, and controlling operation.
2. Description of the Related Art
In the rotors of generators used in energy plant, slots are cut in the axial direction, and coils are inserted into the slots. To prevent the coils from being forced out by centrifugal force, dovetails are provided in the outer periphery of the shaft, into which wedges are inserted.
In generator rotors having this type of fitted structure, as a result of centrifugal forces during operation the wedges are forced against the dovetails of the shaft, and at the same time the rotor is subject to repeated bending due to its self-weight. At the ends of the areas of contact between the shaft dovetails and wedges repeated minute relative slippage, or the phenomenon known as fretting, occurs. If damage due to fretting is severe, it can lead to fatigue, so methods have been proposed to prevent the occurrence of fretting fatigue cracking where the wedges contact the shaft of the generator.
For example, in Japanese Patent Publication No. H4-29304 stress relieving grooves are provided in the contact areas of the shaft corresponding to the joints with the wedges to reduce the contact line stresses in the contact end areas. Also, in for example Japanese Patent Application Laid-open No. H6-197485, by reducing the hardness at the end portions and changing the shape of the wedges, the surface pressure concentration in the contact end areas is relieved. By these measures, if the stresses are reduced in the cover end portions, the potential for occurrence of fretting fatigue cracking can be reduced.
On the other hand, the measures taken are not sufficient, so there have been cases where the occurrence of cracking has been discovered by inspection in the contact area between the shaft dovetail and the wedges. In rotating machinery, fatigue cracks do not remain stationary, but rapidly extend due to the high cycle fatigue accompanying rotation. Therefore the defect in the rotating body is chipped and removed as a rule, regardless of the position of occurrence, shape, or dimension.
For fretting fatigue cracks in the shaft dovetails of generator rotors also, defects discovered during periodic inspection are chipped and removed, similar to other rotating bodies. However, the cracks discovered during periodic maintenance are mostly short cracks near the surface. Also cases where this cracking has ultimately led to the failure of the rotor are rare. Therefore, it is considered that the cracks do not necessarily quickly extend.
In the phenomenon of fretting fatigue cracking of shaft dovetails, high contact line stresses caused by friction forces are concentrated in a very narrow part of the contact end surface. Proceeding to the interior of the surface the stresses rapidly reduce. Also, the mean stress that is acting is compressive, due to the high contact surface pressure, so it is known that the cracks can easily remain stationary.
Also, the mechanism of occurrence of fretting fatigue in shaft dovetails is understood to be as follows, Repeated relative slippage with the wedges is caused by bending of the rotor due to the self-weight of the rotor. This causes repeated friction forces to be generated, which generates a repeated stress amplitude in the surface layer. In the case of this generation mechanism, if the same type of machinery is properly assembled, there should be no difference in the value of repeated stress amplitude caused by friction or the value of shaft dovetail compressive mean stress due to the surface pressure of the wedge pressing against the shaft.
However, the size of cracks actually discovered in the shaft dovetails of generator rotors varies. Even in the same type of machine the same crack size is not necessarily found, so the mechanism of generation of fretting fatigue described above is not sufficient to explain the actually observed phenomena.
Therefore, as it is not possible to quantitatively explain the mechanism of fretting fatigue cracking in shaft dovetails at present, it is not possible to determine whether a fretting fatigue crack in the shaft dovetail of a generator rotor is stationary or not. Therefore, defects that are discovered are chipped and removed. It is also possible to make a design change that changes the mechanical state of the contact area. However, for this it is necessary to temporarily remove the wedge from the dovetail, so there are more operations than chipping and removing the crack.